Aircraft operations are mostly conducted in harsh service conditions, so great attention is paid to the structural health of aircraft components. Most aircraft components are subjected to cyclical loads that induce fatigue failures, leading to microdefect formations, crack propagations, and eventual fracture. Statistics show that more than 60% of service failures in aircraft components occur due to fatigue. This chapter investigates fatigue failure in aircraft materials. First, fatigue concepts are briefly reviewed, and then parameters affecting fatigue failures are discussed.